Shower smells when tub/shower drains in new home, previous owner inept moron

My wife and I purchased our first home a few months back and have run into an issue that has prevented us from using our master shower. Shortly after using the tub or shower the shower starts to get a peculiar odor, my wife thinks it's mold and I think it's more of a moist earthy smell. We have a jetted tub, so I took off the side panel to scope out what I could. The smell inside is pretty similar to what the shower smells like, so I dug up the pipes a bit and and realized how terrible the previous owner was at DIY projects. My brother and I removed the tub and here's what we have -



At the top is the tub drain + overflow, coming from the right is the shower drain, across from that is a "vent" that basically just ended in the open space under the tub (genius), and it all goes down into a shared p-trap that runs to the main drain. My best guess is when it all drains, it siphons musty air from under the tub and the air seeps into the shower. The main vent is about 6 feet down the line from this whole mess. We poured water down to check the p-trap, and it appears to drain smoothly. Do I need to run a vent up to connect it to the main, or can I block off the "vent to nowhere" and let the shower and tub use each other as wet vents? Or is this whole mess in need of an overhaul?

The tub and shower require separate and individual traps. That conglomeration needs to come out.

Well there you go cant fix stupid lol

In repairing problems such as yours, I find my favorite tool is a Sawzall. Cut out that whole mess and start from scratch.

Okay, we took it all out and we're gonna run a 2" p-trap from the shower and a 1½" from the tub that meet up at a 45° wye connected to the main drain(2"). I have two questions. First, should we get a wye that combines the 1½" and 2" pipes, or should we widen the smaller one right before the wye? Second, the main vent is about 6 feet away from where the traps will be and the main drain size is 2", and from what I can tell I shouldn't need to add any venting so long as the slope is 1/4", correct?